The present invention relates to growing thin films on a surface of a substrate. More particularly, the invention concerns an improved method and apparatus for producing a thin film onto a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants.
Conventionally, thin films are grown out using a vacuum evaporation deposition. Molecular Beam Epitaxy (MBE) and other similar vacuum deposition techniques, different variants of Chemical Vapor Deposition (CVD) (including low-pressure and metallo-organic CVD and plasma-enhanced CVD) or, alternatively, the above-mentioned deposition process based on alternate surface reactions, known in the art as the Atomic Layer Deposition, in the following abbreviated ALD, formerly also called Atomic Layer Epitaxy or xe2x80x9cALExe2x80x9d. Equipment for the ALD process is supplied under the name ALCVD(trademark) by ASM Microchemistry Oy, Espoo, Finland.
In the MBE and CVD processes, besides other variables, the thin film growth rate is also affected by the concentrations of the provided starting material inflows. To achieve a uniform surface smoothness of the thin films manufactured using these methods, the concentrations and reactivities of the starting materials must be kept equal over the whole surface area of the substrate. If the different starting materials are allowed to mix which each other prior to reaching the substrate surface, as is the case in the CVD method, the possibility of mutual reactions between the reagents is always imminent. Herein arises a risk of microparticle formation already in the infeed lines of the gaseous reactants. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. However, the occurrence of premature reactions in MBE and CVD reactors can be avoided, e.g., by heating the reactants not earlier than only at the substrates. In addition to heating, the desired reaction can be initiated with the help of e.g., plasma or other similar activating means.
In MBE and CVD processes, the growth rate of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the thin film growth rate in the ALD process is controlled by the substrate surface properties, rather than by the concentrations or other qualifies of the starting material inflows. In the ALD process, the only prerequisite is that the starting material is provided in a sufficient concentration to saturate the surface of the substrate.
The ALD method is described, e.g., in FI Pat. Nos. 52,359 and 57,975 as well as in U.S. Pat. Nos. 4,058,430 and 4,389,973. Also in FI Pat. Nos. 97,730, 97,731 and 100,409 are disclosed some apparatus constructions suited for implementing the method. Equipment for thin film deposition are further described in publications Material Science Report 4(7), 1989, p. 261, and Tyhjixc3x6tekniikka (title in English: Vacuum Techniques), ISBN 951-794-422-5, pp. 253-261. These references are incorporated herein by reference.
In the ALD method, atoms or molecules sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon.
According to the conventional techniques known from FI Patent Specification No. 57,975, the saturation step is followed by a protective gas pulse forming a diffusion barrier that sweeps away the excess starting material and the gaseous reaction products from the substrate. Intermixing of the successive reactant pulses must be avoided. The successive pulses of different starting materials and the protective gas pulses forming diffusion barriers that separate the successive starting materials pulses from each other accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials.
The pulsing of the gaseous reactants and the purge gas is typically controlled by valves.
An essential feature of the ALD process is that condensation of the reactant should be avoided in the vicinity of the reaction chamber. Condensation of the reactant in particular in the conduit between the reactant source and the reaction chamber and on the substrate in the reaction chamber will seriously impair the quality of the thin film. Particles or droplets condensed or sublimed in the reactant feed lines may disperse into the reactant flow and cause inhomogenity on the thin film. The same applies to condensation of solid particles or liquid droplets on the thin film in the reaction chamber. Therefore, an ALD process is operated in such a manner that the temperature in the equipment interconnecting the reactant source and the outlet of the reaction chamber (the xe2x80x9chot zonexe2x80x9d) is not allowed to drop below the condensation temperature of the reactant.
The temperature of the ALD process is determined by the reactants used and by the applied pressure. Generally it lies in the range between the evaporation temperature and the decomposition temperature of the reactant. Usually the temperature is about 25 to 500xc2x0 C. There is a distinct trend toward the use of less volatile reactants such as solid or high-boiling precursors. Such reactant sources are easier to handle. However, the applicable temperature range is distinctly higher for these than for the gaseous and liquid reactants. Usually solid sources are used at temperatures in the range of 250 to 500xc2x0 C., typically 300-450xc2x0 C. The pressure range is typically about 1 to 100 mbar, preferably less than 50 mbar.
When solid reactant sources are used, a carrier gas typically has to be employed for feeding the reactant vapours into the reaction chamber because the vapour pressure of the source is not always sufficient to allow for a sufficiently strong flow of vapour-phase reactant pulses from the source to the reaction chamber. Since many of the solid sources are powders containing extremely finely divided matter (dust), there is a risk for contamination of the vapour-phase reactant pulses with small solid particles when the carrier gas flow is conducted through the reactant material. These particles disturb the growth of the thin film. Similar problems are encountered with liquid reactants having high boiling points in that the flow of the carrier gas may create a mist with finely divided droplet dispersed in the carrier gas flow. Therefore, the vapour-phase reactant pulses may have to be conducted to a purifier, preferably a static purifier, to remove any liquid or droplets or solid particles present in the gas stream, before the pulses are fed into the reaction chamber. Such purifiers may comprise traditional filters in which the gas stream is conducted through a layer of a porous material having macromolecular pores.
Thus, the basic prerequisites of ALD, via operation above the condensation point of the reactant using reactants which are free from particles or droplets which may disturb the homogeneous growth of the thin film, in combination with the trend towards using reactants having high boiling or sublimation points, gives rise to ever more stringent requirement on the ALD equipment. It is necessary to design the apparatus for reliable operation at high temperatures, typically in the range of about 250 to 500xc2x0 C., at reduced pressure. Not only should the equipment used in the xe2x80x9chot zonexe2x80x9d withstand these temperatures as such, but the materials should also be resistant to the action of the reactive vapourised reactants at said temperatures. These conditions are particularly demanding for mechanical valves conventionally employed for, e.g., pulsing of the reactants and purge gas, and for the gaskets and packings of said valves and other fittings. Attrition of the polymer materials used in the gaskets and packings will cause an additional dusting problem resulting in contamination of the vapour-phase reactant pulses. For several reasons it is therefore necessary to incorporate static purifiers into the ALD equipment designed for the use of solid and/or liquid sources.
It is an object of the present invention to provide a novel method of growing a thin film onto a substrate placed in a reaction chamber according to the ALD process. In particular, it is an object of the invention to provide a method in which the ALD process can be operated using solid or liquid reactant sources and employing a purifier for removing solid particles or liquid droplets emanating from, e.g. the precursor sources, while minimizing the costs and the wear of the process equipment.
These and other objectives, together with the advantages thereof over known processes which shall become apparent from the following specification, are accomplished by the invention as hereinafter described and claimed.
Generally, the present invention is based on the idea that mechanical valves conventionally used for regulating the pulsing of the reactants, i.e. the flow of reactants from precursor sources to the reaction chamber, are replaced by gas flow barriers formed by inert or inactive gas in the conduit interconnecting the reactant source with the reaction chamber. These gas barriers are generated in the time interval between two successive pulses of the same reactant gas. The time interval typically includes a purge pulse, a pulse of another reactant and a further purge pulse.
In practice the invention can be implemented by feeding inactive gas into said interconnecting conduit, which in the following will be called the xe2x80x9cfirst conduit,xe2x80x9d via a second conduit, connected to the first conduit at a connection point. The inactive gas is then withdrawn from the first conduit via a draining conduit (in the following the xe2x80x9cthirdxe2x80x9d conduit) connected to the first conduit. The third conduit by-passes the reactor and it is maintained at a temperature equal to or higher than the condensation of the vapour-phase reactant. By connecting the third conduit to the first conduit at a point upstream of the connection point of the second conduit it becomes possible to form a gas phase barrier which is opposite directed to the flow of vaporised reactants from the reactant source via the first conduit into the reaction chamber.
Considerable advantages are obtained by means of the invention. Thus, by means of the present process moving mechanical parts can be avoided in all or most of the area operated at a temperature above the evaporation temperature. The operation of the present regulating mechanism, in the following also called xe2x80x9cinert gas valvingxe2x80x9d is reliable and it is not sensitive to variations in the chemical character of the precursors. Since it includes no moving parts, the investment costs and the need for maintenance work is strongly reduced. As will be discussed in more detail below, by the inert gas valving system, pulsing of reactants can be carried out by using only one valve which controls the flow of carrier gas from the source of inactive or inert gas to the precursor source. This valve can be kept at ambient temperature and it is not in direct contact with the reactants. By maintaining the temperature of the draining conduit above the evaporation temperature of the reactant, condensation of the reactant in the hot zone of the apparatus can be avoided. There is no build-up of condensated reactants in the third conduit during the purse phase. All parts of the equipment are kept cleaner and there are formed less particles which could be forwarded to the reaction chamber. Precursor waste during the purge cycle can be minimized by providing for static gas flow conditions in the source.
According to a preferred embodiment, wherein the purifier is incorporated into the conduit interconnecting the reactant source with the reaction chamber, the inert gas barrier is arranged downstream of the purifier whereby there is a one-way flow of gas over the purifier during the whole operation of the ALD process. Since the flow direction over and through the purifier does not change, the risk of particles and droplets absorbed in the purifier being released therefrom is eliminated.
It should be noted that certain objects and advantages of the invention have been described above for the purpose of describing the invention and the advantages achieved over the prior art. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
It should also be noted that all of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclose